Group III nitride semiconductors are compound semiconductor materials obtained as compounds of any of aluminum (Al) atoms, gallium (Ga) atoms, and indium (In) atoms, which are Group IIIB elements (hereinafter, simply III elements), and nitrogen (N) atoms, which are a Group VB element (hereinafter, simply Group V element), i.e., aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN), as well as mixed crystals thereof (AlGaN, InGaN, InAlN, and InGaAlN). Such Group III nitride semiconductors are materials expected to be applied to optical elements such as light emitting diodes (LEDs), laser diodes (LDs), photovoltaic solar cells (PVSCs), and photo diodes (PDs) that cover a wide wavelength range from a far ultraviolet range to a visible range to a near infrared range, as well as to electronic elements such as high electron mobility transistors (HEMTs) and metal-oxide-semiconductor field effect transistors (MOSFETs) for high-frequency, high-output use.
In general, in order to implement applications as described above, it is necessary to epitaxially grow a Group III nitride semiconductor thin film on a single-crystal substrate to obtain a high-quality single-crystal film (epitaxial film) with few crystal defects. To obtain such an epitaxial film, it is most desirable to perform homo-epitaxial growth by using a substrate made of the same material as the epitaxial film.
However, a single-crystal substrate made of a Group III nitride semiconductor is extremely expensive and has not therefore been utilized, except in some applications. Instead, a single-crystal film is obtained by hetero-epitaxial growth on a substrate of a different kind of material which is mainly sapphire (α-Al2O3) or silicon carbide (SiC). In particular, α-Al2O3 substrates are inexpensive, and those with a large area and a high quality are available. Thus, α-Al2O3 substrates are utilized in almost all the commercially-available LEDs using Group III nitride semiconductor thin films.
Meanwhile, the epitaxial growth of a Group III nitride semiconductor thin film as described above uses metalorganic chemical vapor deposition (MOCVD) which can provide an epitaxial film with a high quality and productivity. However, MOCVD has problems such as requiring a high production cost and having a tendency to develop particles, making it difficult to achieve a high yield.
In contrast, sputtering has characteristics of being capable of saving the production cost and having a low probability of developing particles. Accordingly, if at least part of the process for forming a Group III nitride semiconductor thin film can be replaced with sputtering, it may be possible to solve at least part of the above problems.
However, Group III nitride semiconductor thin films fabricated by sputtering have a problem that their crystal qualities tend to be poorer than those fabricated by MOCVD. For example, NPL 1 discloses the crystallinity of a Group III nitride semiconductor thin film fabricated by using sputtering. According to the description of NPL 1, a c-axis oriented GaN film is epitaxially grown on an α-Al2O3 (0001) substrate by using radio-frequency magnetron sputtering, and the full width at half maximum (FWHM) of X-ray rocking curve (XRC) measurement on GaN (0002) plane is 35.1 arcmin (2106 arcsec). This value is a significantly large value as compared to GaN films on α-Al2O3 substrates that are currently commercially available, and indicates that tilt mosaic spread, which will be described layer, is large and the crystalline quality is poor.
In other words, in order to employ sputtering as a process for forming a Group III nitride semiconductor thin film, it is necessary to reduce the mosaic spread of an epitaxial film made of a Group III nitride semiconductor so that a high crystalline quality can be achieved.
Meanwhile, there are tilt mosaic spread (offset of the crystalline orientation in a direction perpendicular to the substrate) and twist mosaic spread (offset of the crystalline orientation in an in-plane direction) as indexes to indicate the crystalline quality of an epitaxial film made of a Group III nitride semiconductor. FIGS. 10A to 10D are schematic views of crystals made of a Group III nitride semiconductor and epitaxially grown in the c-axis direction on an α-Al2O3 (0001) substrate. In FIGS. 10A to 10D, reference numeral 901 is the α-Al2O3 (0001) substrate; 902 to 911, the crystals made of the Group III nitride semiconductor; cf, the orientation of the c axis of each crystal made of the Group III nitride semiconductor; cs the orientation of the c axis of the α-Al2O3 (0001) substrate; af, the orientation of the c axis of each crystal made of the Group III nitride semiconductor; and as, the orientation of the a axis of the α-Al2O3 (0001) substrate.
Here, FIG. 10A is a bird's eye view showing how crystals made of the Group III nitride semiconductor are formed while having a tilt mosaic spread, and FIG. 10B shows the cross-sectional structures of part of the crystals. As can be seen from these drawings, the orientation cf of the c axis of each of crystals 902, 903, and 904 made of the Group III nitride semiconductor is substantially in parallel to the orientation cs of the c axis of the substrate, and is the most dominant crystalline orientation in the direction perpendicular to the substrate. On the other hand, each of crystals 905 and 906 made of the Group III nitride semiconductor is formed such that the orientation cf of its c axis is slightly off the dominant crystalline orientation in the direction perpendicular to the substrate. Moreover, FIG. 10C is a bird's eye view showing how the crystals made of the Group III nitride semiconductor are formed while having a twist mosaic spread, and FIG. 10D shows a plan view thereof. As can be seen from these drawings, the orientation af of the a axis of each of crystals 907, 908, and 909 made of the Group III nitride semiconductor is the most dominant crystalline orientation in an in-plane direction because their angles with respect to the orientation as of the a axis of the α-Al2O3 (0001) substrate are all 30° approximately. On the other hand, each of crystals 910 and 911 made of the Group III nitride semiconductor is formed such that the orientation af of its a axis is slightly off the dominant crystalline orientation in the in-plane direction.
Offset from the most dominant crystalline orientation as described above is called mosaic spread. Specifically, offset of a crystalline orientation in the direction perpendicular to the substrate is referred to as tilt mosaic spread, while offset of a crystalline orientation in an in-plane direction is referred to as twist mosaic spread. It is known that tilt and twist mosaic spreads are correlated to the density of defects formed inside a Group III nitride semiconductor thin film such as screw dislocations and edge dislocations. By reducing tilt and twist mosaic spreads, the density of defects described above is reduced, thus making it easier to obtain a high-quality Group III nitride semiconductor thin film.
Note that the levels of tilt and twist mosaic spreads can be evaluated by checking the FWHM of a diffraction peak obtained by XRC measurement on a specific lattice plane (symmetrical plane) formed in parallel to the substrate surface or on a specific lattice plane formed perpendicular to the substrate surface.
Note that FIGS. 10A to 10D and the above description are intended to describe tilt and twist mosaic spreads through a simple, conceptual approach, and not to guarantee any specificity. For example, it is not always the case that the above-described most dominant crystalline orientation in the direction perpendicular to the substrate and the above-described most dominant crystalline orientation in the in-plane direction coincide completely with the orientations of the c axis and the a axis of the α-Al2O3 (0001) substrate. Further, it is not always the case that a gap between two crystals as shown in FIG. 10D is formed. What is important is that mosaic spread indicates the degree of offset from a dominant crystalline orientation.
Meanwhile, in general, Group III nitride semiconductor thin films include a +c polarity growth type and a −c polarity growth type as shown in FIG. 11. It is known that a fine epitaxial film is more likely to be obtained by the +c polarity growth than by the −c polarity growth. Thus, it is desirable to obtain a +c polarity epitaxial film in addition to employing sputtering as a process for forming a Group III nitride semiconductor thin film.
It is to be noted that in this description, “+c polarity” is a term meaning Al polarity, Ga polarity, and In polarity for AlN, GaN, and InN, respectively. Moreover, “−c polarity” is a term meaning N polarity.
Heretofore, a number of approaches have been made to obtain a fine Group III nitride semiconductor thin film (see PTLs 1 and 2).
PTL 1 discloses a method in which an α-Al2O3 substrate is subjected to plasma processing before a Group III nitride semiconductor thin film (AlN in PTL 1) is formed on the substrate by using sputtering so that the Group III nitride semiconductor thin film can achieve a high quality, i.e., a Group III nitride semiconductor thin film with a significantly small tilt mosaic spread, in particular, can be obtained.
Moreover, PTL 2 discloses a method of manufacturing a Group III nitride semiconductor (a Group III nitride compound semiconductor in PTL 2) light emitting element, in which a buffer layer (an intermediate layer in PTL 2) made of a Group III nitride semiconductor (a Group III nitride compound in PTL 2) is formed on a substrate by sputtering, and then an n-type semiconductor layer including an underlying film, a light emitting layer, and a p-type semiconductor layer are sequentially stacked on the buffer layer made of the Group III nitride semiconductor.
In PTL 2, the procedure for forming the buffer layer made of the Group III nitride semiconductor is described as including: a pre-processing step of performing plasma processing on the substrate; and a step of forming the buffer layer made of the Group III nitride semiconductor by sputtering after the pre-processing step. Moreover, in PTL 2, an α-Al2O3 substrate and AlN are used as preferred forms of the substrate and the buffer layer made of the Group III nitride semiconductor, respectively, and MOCVD is preferably used as the method of forming the n-type semiconductor layer including the underlying film, the light emitting layer, and the p-type semiconductor layer.